A main circuit of a power conversion apparatus for driving an AC motor is composed of a converter which rectifies AC power from a system power supply, and an inverter which performs reconversion to AC power suitable for the AC motor, and a smoothing capacitor is connected to a DC link which is a DC link section between the converter and the inverter. When the power conversion apparatus is connected to the system power supply, an LC resonance circuit is formed by a reactor of the system power supply and the smoothing capacitor. In a three-phase diode converter, it is known that, along with rectification operation, a pulsation having six times of the power supply frequency occurs on the DC output side. Therefore, if the resonance frequency coincides with six times of the system power supply frequency, the DC link voltage in an inverter main circuit largely oscillates. As a result, breakage of main circuit components can occur or AC motor control can become unstable.
Particularly, in the case of using a smoothing capacitor having a small capacitance, the resonance frequency with the reactor increases, often resulting in the above problem. In response to the above problem, techniques directed to resonance suppression are proposed.
For example, Patent Document 1 employs a method of: extracting a pulsation component of the DC link voltage; instead of obtaining a differential amount of the oscillation, obtaining a variation amount of the oscillation per a predetermined time by a method using pseudo-differential for generating an amount for phase leading; and correcting the frequency of the inverter output voltage by using the obtained signal, thereby suppressing the pulsation of the DC link voltage. Then, the phase leading processing of generating a differential signal of the DC link voltage is designed such that, particularly, a specific phase leading amount occurs on a frequency component six times as high as the power supply frequency.
However, if the resonance suppression is performed, torque ripple of the AC motor increases. Therefore, for example, in Patent Document 2, the resonance suppression is performed only when an LC resonance frequency coincides with six times of the power supply frequency. When they do not coincide with each other, the resonance suppression is not performed, whereby increase in the torque ripple can be prevented as much as possible.